1. Field of the Invention
The present invention relates generally to data backup systems for use with computers and more particularly to an apparatus and method for backing up a storage system to tape while the storage system is in use.
2. Description of the Prior Art
Data processing systems commonly use one or more disk drives for storage of data and programs. The host computer will retrieve from the disk system the particular information currently required and will send to the disk system for storage new or updated information or data or programs which the host may require in the future, but does not require in internal main memory at that time.
Many organizations, such as large businesses and governmental entities, have extremely large databases of information which they need to have readily available for rapid access and modification. These databases may in some circumstances equal or exceed one terabyte of data and require large data storage systems containing multiple disk drives or arrays of disk drives organized into a single large logical memory system. Often there is a host processor, or server, that is dedicated solely or primarily to handling database transactions. The various users of the database transmit their database requests to the database server and receive the requested data from the database server via a network.
Organizations using such large databases typically need to create backups of their databases from time to time for business, legal or archival reasons. Also, while modern disk systems are, in general, highly reliable devices, some organizations may desire to have their database backed up as protection against the possibility of a storage system failure.
It is, therefore, a common practice to periodically perform a backup of part or all of the data on the disk system. Typically this is done by copying onto magnetic tapes the data desired to be backed up. The tapes are then retained for a period of time, as determined by the system user, and can be used to restore the system to a known condition, if necessary.
A number of commercial utility programs are available for performing backup operations. Typically, these utilities are intended to run on the database server. In some cases, the utility can be run on another computer system which communicates with the database server via the LAN. This has drawbacks in that backing up a terabyte database over a LAN would be very slow and, whether the backup utility is running on the backup server or on another computer on the LAN, the participation of the database server during the backup process is required. Involving the database server will divert processing power away from the primary- tasks of the server and may either degrade the response time to system users or lengthen the time required to complete the backup.
Another problem in the prior art is that organizations generally desire a "snapshot" of their database as it exists at a certain point in time. One means of ensuring data consistency during the backup is to restrict users from having access to the data during the backup operation. Since the backup for extremely large databases can sometimes take hours, it is often unacceptable to the organizations for their databases to be unavailable to their users for the duration of the backup.
Prior art systems have been developed in an effort to resolve this problem and allow users to continue to write to the database while the backup is in progress. For example, U.S. Pat. No. 5,535,381 discloses a system wherein a copy-on-write (COW) technique is used to save "original" data to separate buffers prior to execution of a write command. The COW data is stored on the backup tape in separate tape records, so the image stored on the tape is not a duplicate of the original image and requires reintegration to recreate the original data image.
U.S. Pat. No. 5,487,160 discloses another COW technique wherein original data is stored on a spare disk drive until the backup is complete and then the contents of the spare drive is transferred to the backup tape in bulk. Here again, the image on the backup tape is fragmented and requires a reintegration process to reconstitute the original image.
The present invention resolves these problems and drawbacks by allowing users unrestricted access to the system during the backup process while creating a snapshot backup image on tape that does not require reconstruction.